1. Field of the Invention
The present invention relates to a polycarbonate resin for use in the production of a substrate for an optical information medium. More particularly, the present invention is concerned with-a substantially chlorine-free polycarbonate resin which is produced by subjecting an aromatic dihydroxy compound and a carbonic diester to a transesterification reaction and which comprises a plurality of aromatic polycarbonate main chains which collectively contain specific heterounits (A) and (B), wherein the weight average molecular weight of the polycarbonate resin is from 13,000 to 18,000. The polycarbonate resin of the present invention exhibits a high melt fluidity during the injection molding thereof. By virtue of this property, the polycarbonate resin of the present invention can be advantageously used for producing an excellent substrate for an optical information medium, wherein the substrate has not only excellent optical properties, such as a low birefringence, but also a high precision transferability of information (that is, information of a stamper (a mold for forming microgrooves and micropits on a substrate) can be transferred to the substrate with high precision) (such a property is referred to as xe2x80x9chigh precision transferabilityxe2x80x9d). Such a substrate can be advantageously used as a substrate for an optical information medium having a high recording density, such as a DVD. The present invention is also concerned with a polycarbonate resin composition comprising the above-mentioned polycarbonate resin-and an acidic compound, and a substrate for an optical information medium, which is produced by molding the above-mentioned polycarbonate resin or the above-mentioned polycarbonate resin composition.
2. Prior Art
Polycarbonates have been widely used in various fields as engineering plastics having excellent heat resistance, impact resistance and transparency. Due to the recent expansion of information society, there has been a growing demand for polycarbonates to be used for producing storage media for music and image, and storage media for digital information (such as a storage medium for a personal computer). Nowadays, polycarbonates have become indispensable resins for producing optical disks and optical cards, such as a CD, a CD-ROM, CD-R, a DVD-ROM, a DVD-R, an MO and an MD.
In the production of a substrate for optical information medium, such as an optical disk, precise formation of microgrooves and micropits on the substrate is necessary. Polycarbonates for use in the production of such a substrate need to have a high transferability and excellent optical properties, such as a low birefringence. Therefore, low molecular weight polycarbonates having a weight average molecular weight of about 15,500 and having a high melt fluidity are used in the production of the substrates for the optical information media. In recent years, with respect to the storage media, conventional CD""s are being replaced by DVD""s which have a recording density much higher than the CD""s. For producing substrates used in such optical information media having a high recording density, it is necessary to form very precise microgrooves and micropits on the substrate. Accordingly, as a resin for use in the production of a substrate for the optical information medium, the development of a polycarbonate which has a transferability higher than that of the conventional polycarbonates is demanded. In addition, due to the fact that the thickness of a substrate for a DVD (DVD substrate) is only about 0.6 mm which is very small as compared to the thickness (about 1.2 mm) of a substrate for a CD (CD substrate), there is also a growing demand for the development of a polycarbonate having a melt fluidity higher than that of the conventional polycarbonates. When a DVD substrate is produced from a polycarbonate, which has conventionally been used for producing the CD substrates, at a molding temperature employed in the conventional production of the CD substrates (i.e., about 300 to 320xc2x0 C.), the melt fluidity of the polycarbonate is insufficient to obtain a satisfactory level of transferability. Therefore, in order to solve this problem, the DVD substrates are produced at a molding temperature as high as 380 to 390xc2x0 C. so as to lower the melt viscosity of the polycarbonate. Such a molding temperature is too high and, hence, is not generally used for molding polycarbonates (the molding temperature in the production of molded articles other than optical information media is generally from 270 to 300xc2x0 C., and the molding temperature in the production of the CD substrates is about 320xc2x0 C.).
The use of such a high molding temperature causes problems, such as heat deterioration of a polycarbonate, long molding cycle, and large warpage of the produced substrate.
Further, with respect to the optical information media, there is especially a large demand for a storage medium capable of retaining the stored information for a prolonged period of time. Stability under conditions wherein both temperature and humidity are high is required for a storage medium to be able to retain the stored information for a prolonged period of time.
In the production of the substrates for the optical information media, polycarbonates produced by the phosgene process have conventionally been used. However, the use of the polycarbonates produced by the phosgene process in the production of the substrates for the optical information media is accompanied by the following problems: (1) phosgene used in this process is difficult to handle; (2) methylene chloride used as a solvent in this process causes polycarbonates to contain impurities (chlorine ions and residual methylene chloride) which not only adversely affects the thermal stability of the polycarbonates, but also causes corrosion of a mold used for the molding of the polycarbonates; and (3) the quality of the produced substrate for the optical information medium is poor.
In order to alleviate these problems accompanying the phosgene process polycarbonates, various proposals for polycarbonate compositions containing decreased amounts of the above-mentioned impurities and methods for decreasing the amounts of the impurities contained in the polycarbonates have been made in a number of documents, including Unexamined Japanese Patent Application Laid-Open Specification No. 63-316313 (corresponding to US Patent No. 4,880,896), Unexamined Japanese Patent Application Laid-Open Specification No. 4-146922, and Unexamined Japanese Patent Application Laid-Open Specification No. 63-97627 (corresponding to U.S. Pat. No. 4,798,767). However, when the molding of polycarbonates is conducted-at a high temperature, methylene chloride remaining in a small amount in the polycarbonates is easily converted into hydrochloric acid. The removal of impurities by the conventional methods is unsatisfactory and, hence, it has been desired to develop a method for almost completely removing the impurities from polycarbonates.
On the other hand, since the complete removal of impurities from polycarbonates requires a great deal of labor, polycarbonates produced by the transesterification process, which uses no phosgene or methylene chloride, have recently been drawing attention.
However, the transesterification polycarbonates have the following problem. The transesterification polycarbonates contain a large amount of hydroxyl terminals as compared to the phosgene process polycarbonates (see xe2x80x9cKobunshi Bunseki Handbook (Handbook for Polymer Analysis)xe2x80x9d, page 345, published in 1985 by Asakura Shoten, Japan). Such polycarbonates suffer severe heat deterioration during the high temperature molding and, hence, cannot be used for producing a substrate for the optical information medium.
At the present, there is no transesterification polycarbonate which can be used for producing a substrate for the optical information medium, especially a high recording density medium, such-as DVD, the production of which requires a molding at a high temperature. Therefore, the development of such a transesterification polycarbonate has been desired in the art.
In this situation, the present inventors have made extensive and intensive studies with a view toward solving the above-mentioned problems. As a result, it has been found that a substantially chlorine-free polycarbonate resin which is produced by subjecting an aromatic dihydroxy compound and a carbonic diester to a transesterification reaction, and which comprises a plurality of aromatic polycarbonate main chains which collectively contain specific heterounits (A) and (B), wherein the weight average molecular weight of the polycarbonate resin is from 13,000 to 18,000, can be advantageously used in the production of a substrate for an optical information medium. Specifically, it has been found that such a polycarbonate resin exhibits a high melt fluidity during the injection molding thereof and, hence, can be used for producing a substrate for an optical information medium, which has not only excellent optical properties, such as a low birefringence, but also a high precision transferability (i.e., information of a stamper (a mold for forming microgrooves and micropits on a substrate) is transferred to the substrate with high precision). Such a substrate can be advantageously used as a substrate for an optical information medium having a high recording density, such as a DVD.
Further, the present inventors have also found that, when a polycarbonate resin composition prepared by adding an acidic compound to the above-mentioned polycarbonate resin is molded, the occurrence of optical microdefects (craze-like defects having a maximum diameter of less than 200 xcexcm which scatter or block the light) in the molded article can be satisfactorily suppressed even under conditions wherein both temperature and humidity are high.
The present invention has been completed, based on the above findings.
Accordingly, it is an object of the present invention to provide a polycarbonate resin which exhibits a high melt fluidity during the injection molding thereof and, hence, can be used for producing an excellent substrate for a optical medium, wherein the substrate has not only excellent optical properties, such as a low birefringence, but also a high precision transferability (i.e., information of a stamper (a mold for forming microgrooves and micropits on a substrate) is transferred to the substrate with high precision), so that the substrate can be advantageously used as a substrate for an optical information medium having a high recording density, such as a DVD.
It is another object of the present invention to provide a polycarbonate resin composition comprising the above-mentioned polycarbonate resin and an acidic compound.
It is still another object of the present invention to provide a substrate for an optical information medium, which is obtained by molding the above-mentioned polycarbonate resin or the above-mentioned polycarbonate resin composition.
The foregoing and other objects, features and advantages of the present invention will be apparent from the following detailed description and appended claims.
According to the present invention, there is provided a polycarbonate resin for use in the production of a substrate for an optical information medium, which is produced by subjecting an aromatic dihydroxy compound and a carbonic diester to a transesterification reaction, and is substantially free of a chlorine atom,
the polycarbonate resin comprising a plurality of aromatic polycarbonate main chains, each comprising recurring units each independently represented by the following formula (1): 
xe2x80x83wherein Ar represents a divalent C5-C200 aromatic group,
wherein the plurality of aromatic polycarbonate main chains collectively contain therein at least one heterounit (A) and at least one heterounit (B),
heterounit (A) being represented by the following formula (2): 
xe2x80x83wherein Arxe2x80x2 represents a trivalent C5-C200 aromatic group, and X represents a polycarbonate chain having recurring units each represented by the formula 
xe2x80x83wherein Ar is as defined above and having a molecular weight of from 214 to 6,000, and
wherein, when the polycarbonate main chains contain a plurality of heterounits (A), heterounits (A) are the same or different,
heterounit (B) being represented by the following formula (3): 
xe2x80x83wherein Arxe2x80x2 is as defined above, and wherein, when the polycarbonate main chains contain a plurality of heterounits (B), heterounits (B) are the same or different,
wherein X in formula (2) optionally contains at least one heterounit selected from the group consisting of heterounits (A) and (B),
the sum of the amounts of heterounit (A) and heterounit (B) being from 0.03 to 0.3 mole %, based on the total molar amount of recurring units (1),
the amount of heterounit (A) being at least 50 mole %, based on the total molar amount of heterounit (A) and heterounit (B),
the polycarbonate resin having a weight average molecular weight of from 13,000 to 18,000.
For easy understanding of the present invention, the essential features and various embodiments of the present invention are enumerated below.
1. A polycarbonate resin for use in the production of a substrate for an optical information medium, which is produced by subjecting an aromatic dihydroxy compound and a carbonic diester to a transesterification reaction, and is substantially free of a chlorine atom, the polycarbonate resin comprising a plurality of aromatic polycarbonate main chains, each comprising recurring units each independently represented by the following formula (1): 
xe2x80x83wherein Ar represents a divalent C5-C200 aromatic group,
wherein the plurality of aromatic polycarbonate main chains collectively contain therein at least one heterounit (A) and at least one heterounit (B),
heterounit (A) being represented by the following formula (2): 
xe2x80x83wherein Arxe2x80x2 represents a trivalent C5-C200 aromatic group, and X represents a polycarbonate chain having recurring units each represented by the formula 
xe2x80x83wherein Ar is as defined above and having a molecular
weight of from 214 to 6,000, and wherein, when the polycarbonate main chains contain a plurality of heterounits (A), heterounits (A) are the same or different,
heterounit (B) being represented by the following formula (3): 
xe2x80x83wherein Arxe2x80x2 is as defined above, and wherein, when the polycarbonate main chains contain a plurality of heterounits (B), heterounits (B) are the same or different,
wherein X in formula (2) optionally contains at least one heterounit selected from the group consisting of heterounits (A) and (B),
the sum of the amounts of heterounit (A) and heterounit (B) being from 0.03 to 0.3 mole %, based on the total molar amount of the recurring units (1),
the amount of heterounit (A) being at least 50 mole %, based on the total molar amount of heterounit (A) and heterounit (B),
the polycarbonate resin having a weight average molecular weight of from 13,000 to 18,000.
2. The polycarbonate resin according to item 1 above, which contains a particulate heat deterioration product in an amount of not more than 20 particles, relative to 100 g of the polycarbonate resin, each particle of the heat deterioration product having a particle size of 5 xcexcm or more and having a heat deterioration degree of 0.2 or more as measured in terms of an absorption intensity ratio A1/A2, wherein A1 represents the absorption intensity at the wave number of 1740 cmxe2x88x921, which is ascribed to the stretching vibration of a carbonyl group contained in the non-carbonate type ester linkage present in the polycarbonate resin, and A2 represents the absorption intensity at the wave number of 1780 cmxe2x88x921, which is ascribed to the stretching vibration of a carbonyl group of the carbonate type ester linkage present in the polycarbonate resin, each absorption intensity being measured by infrared spectroscopy.
3. The polycarbonate resin according to item 1 above, wherein heterounit (A) is present in an amount of from 50 to 95 mole %, based on the total molar amount of heterounit (A) and heterounit (B).
4. The polycarbonate resin according to item 1 above, which has terminal hydroxyl groups in an amount of from 5 to 50 mole %, based on the total molar amount of the terminal groups of the polycarbonate resin.
5. The polycarbonate resin according to item 1 above, wherein 85% o r more of recurring units (1) are each represented by the following formula (1xe2x80x2): 
6. A substrate for an optical information medium, which is produced by molding the polycarbonate resin of item 1 above.
7. A polycarbonate resin composition for use in the production of a substrate for an optical information medium, comprising:
(I) 100 parts by weight of the polycarbonate resin of item 1 above; and
(II) 0.1xc3x9710xe2x88x924 to 100xc3x9710xe2x88x924 parts by weight of an acidic compound.
8. A substrate for an optical information medium, which is produced by molding the polycarbonate resin composition of item 7 above.
The polycarbonate resin of the present invention is a polycarbonate resin which is produced by subjecting an aromatic dihydroxy compound and a carbonic diester to a transesterification reaction, and is substantially free of a chlorine atom. With respect to the structure of the polycarbonate resin of the present invention, the polycarbonate resin comprises a plurality of aromatic polycarbonate main chains, each comprising recurring units each independently represented by formula (1) above, wherein the plurality of aromatic polycarbonate main chains collectively contain therein at least one heterounit (A) and at least one heterounit (B).
Heterounit (A) is represented by formula (2) above, wherein, when the polycarbonate main chains contain a plurality of heterounits (A), heterounits (A) are the same or different. Heterounit (B) is represented by formula (3) above, wherein, when the polycarbonate main chains contain a plurality of heterounits (B), heterounits (B) are the same or different.
In formulae (1), (2) and (3) above, each Ar independently represents a divalent C5-C200 aromatic group, and each Arxe2x80x2 independently represents a trivalent C5-C200 aromatic group which has a structure equivalent to Ar having one hydrogen atom thereof removed. Examples of divalent aromatic groups Ar include a phenylene group, a naphthylene group, a biphenylene group, a pyridylene group and a divalent aromatic group represented by the formula: xe2x80x94Ar1xe2x80x94Qxe2x80x94Ar2xe2x80x94, wherein each of Ar1 and Ar2 independently represents a divalent C5-C70 carbocyclic or heterocyclic aromatic group, and Q represents a divalent C1-C30 alkane group.
In the divalent aromatic groups Ar1 and Ar2, at least one hydrogen atom thereof may be replaced by a substituent such that it does not adversely affect the transesterification reaction for producing a polycarbonate resin, such as a halogen atom, an alkyl group having from 1 to 10 carbon atoms, an alkoxy group having from 1 to 10 carbon atoms, a phenyl group, a phenoxy group, a vinyl group, a cyano group, an ester group, an amide group or a nitro group.
Preferred examples of heterocyclic aromatic groups include an aromatic group having in a skeleton thereof at least one hetero atom, such as a nitrogen atom, an oxygen atom or a sulfur atom.
Examples of divalent aromatic groups Ar1 and Ar2 include an unsubstituted or substituted phenylene group, an unsubstituted or substituted biphenylene group and an unsubstituted or substituted pyridylene group. Substituents for Ar1 and Ar2 are as described above for heterocyclic aromatic groups.
Examples of divalent alkane groups Q include organic groups respectively represented by the following formulae: 
wherein each of R1, R2, R3 and R4 independently represents a hydrogen atom, an alkyl group having from 1 to 10 carbon atoms, an alkoxy group having from 1 to 10 carbon atoms, a cycloalkyl group having from 5 to 10 ring-forming carbon atoms, a carbocyclic aromatic group having from 5 to 10 ring-forming carbon atoms or a carbocyclic aralkyl group having from 6 to 10 ring-forming carbon atoms; k represents an integer of from 3 to 11; each Z represents a carbon atom and has R5 and R6 bonded thereto; each R5 independently represents a hydrogen atom or an alkyl group having from 1 to 6 carbon atoms, and each R6 independently represents a hydrogen atom or an alkyl group having from 1 to 6 carbon atoms; and
wherein at least one hydrogen atom of each of R1, R2, R3, R4, R5 and R6 may be independently replaced by a substituent which does not adversely affect the transesterification reaction for producing a polycarbonate resin, such as a halogen atom, an alkyl group having from 1 to 10 carbon atoms, an alkoxy group having from 1 to 10 carbon atoms, a phenyl group, a phenoxy group, a vinyl group, a cyano group, an ester group, an amide group or a nitro group.
Specific examples of divalent aromatic groups Ar include groups respectively represented by the following formulae: 
wherein each of R7 and R8 independently represents a hydrogen atom, a halogen atom, an alkyl group having from 1 to 10 carbon atoms, an alkoxy group having from 1 to 10 carbon atoms, a cycloalkyl group having from 5 to 10 ring-forming carbon atoms, or an phenyl group; each of m and n independently represents an integer of from 1 to 4, with the proviso that when m is an integer of from 2 to 4, the R7""s are the same or different, and when n is an integer of from 2 to 4, the R8""s are the same or different.
Further examples of divalent aromatic groups Ar include those which are represented by the following formula:
xe2x80x94Ar1xe2x80x94Z xe2x80x2xe2x80x94Ar2xe2x80x94
wherein Ar1 and Ar2 are as defined above; and Zxe2x80x2 represents a single bond or a divalent group, such as xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94SOxe2x80x94, xe2x80x94COOxe2x80x94, and xe2x80x94CON(R1)xe2x80x94, wherein R1 is as defined above.
Specific examples of such divalent aromatic groups Ar include groups respectively represented by the following formulae: 
wherein R7, R8, m and n are as defined above.
In the present invention, these aromatic groups Ar may be used individually or in combination.
As a preferred example of recurring units of formula (1) above in the polycarbonate resin of the present invention, there can be mentioned a unit represented by the above-mentioned formula (1xe2x80x2), which is derived from bisphenol A. It is preferred that 85 mole % or more of the recurring units (1) are the units of formula (1xe2x80x2).
With respect to heterounit (A), it is preferred that heterounit (A) is a unit derived from bisphenol A, which is represented by the following formula (2xe2x80x2): 
wherein X is as defined for formula (2).
With respect to heterounit (B), it is preferred that heterounit (B) is a unit derived from bisphenol A, which is represented by the following formula (3xe2x80x2): 
In the polycarbonate resin of the present invention, it is necessary that the sum of the amounts of heterounit (A) and heterounit (B) be in the range of from 0.03 to 0.3 mole %, preferably from 0.04 to 0.25 mole %, more preferably from 0.05 to 0.20 mole %, based on the total molar amount of recurring units (1). The amount of heterounit (A) is at least 50 mole %, preferably from 50 to 99 mole %, more preferably from 70 to 97 mole %, most preferably from 80 to 95 mole %, based on the total molar amount of heterounit (A) and heterounit (B). When the amount of heterounit (A) is less than the above-mentioned range, the melt fluidity of the polycarbonate resin cannot be satisfactorily improved, so that a high molding temperature is necessary for obtaining a satisfactory melt fluidity. However, as mentioned above, a high molding temperature causes problems, such as heat deterioration of the polycarbonate resin, a long molding cycle, and a large warpage of the molded article. On the other hand, when the amount of heterounit (A) is larger than the above-mentioned range, the polycarbonate resin exhibits an improved melt fluidity; however, a substrate obtained by molding the polycarbonate resin has a low mechanical strength and suffers optical defects under conditions wherein both temperature and humidity are high.
In addition to the above-mentioned heterounits (A) and (B), the polycarbonate resin of the present invention may optionally contain heterounits represented by the following formulae: 
wherein, Ar, Arxe2x80x2 and X are as defined for formulae (1) and (2) above, Arxe2x80x3 represents a tetravalent C5-C200 aromatic group, and Y represents a polycarbonate chain having recurring units each represented by the formula 
xe2x80x83and having a molecular weight of from 214 to 6,000.
When bisphenol A is used as an aromatic dihydroxy compound, it is preferred that the above-mentioned optional heterounits have structures represented by the following formulae: 
wherein X and Y are as defined above.
In the present invention, the determination of each of recurring units (1), and heterounits (A) and (B) can be conducted, for example, by a method in which the polycarbonate resin is completely hydrolyzed, and the resultant hydrolysis mixture is analyzed by reversed phase liquid chromatography (the analysis by reversed phase liquid chromatography can be conducted under the conditions as described below in the Examples). With respect to the hydrolysis of the polycarbonate resin, it is preferred that the hydrolysis is conducted at room temperature by the method described in xe2x80x9cPolymer Degradation and Stabilityxe2x80x9d 45 (1994), 127-137. The hydrolysis by this method is advantageous in that the complete hydrolysis of a polycarbonate resin can be achieved by simple operation without the danger of occurrence of side reactions during the hydrolysis. Further, when the polycarbonate resin contains the above-mentioned optional heterounits, the optional heterounits can also be determined by the above-mentioned method.
The polycarbonate resin of the present invention has a weight average molecular weight of from 13,000 to 18,000, preferably from 13,000 to 17,000, more preferably from 13,500 to 16,000. When the weight average molecular weight is larger than the above-mentioned range, the melt fluidity of the polycarbonate resin becomes unsatisfactory, so that the polycarbonate resin cannot be used for producing a substrate for an optical information medium having a high recording density, such as a DVD. When the weight average molecular weight is smaller than the above-mentioned range, the mechanical strength of the substrate produced by molding the polycarbonate resin becomes unsatisfactory. In the present invention, the weight average molecular weight of the polycarbonate resin can be measured by gel permeation chromatography (GPC) using tetrahydrofuran (as a solvent) and a polystyrene gel column. As a calibration curve, a modified calibration curve for a polycarbonate is used, which is obtain ed by modifying a calibration curve obtained with respect to standard monodisperse polystyrene samples, wherein the modification of the calibration curve is made by a calculation using the following formula:
Mpc=0.3591 Mps1.0388
wherein Mpc represents the molecular weight of the polycarbonate resin, and Mps represents the molecular weight of the standard polystyrene.
Generally, polycarbonate resins contain unfavorouble particulate heat deterioration product. In the present invention, it is preferred that the amount of a particulate heat deterioration product present in the polycarbonate resin is not more than 20 particles, relative to 100 g of the polycarbonate resin, wherein each particle of the heat deterioration product has a particle size of 5 xcexcm or more and has a heat deterioration degree of 0.2 or more as measured in terms of an absorption intensity ratio A1/A2, wherein A1 represents the absorption intensity at the wave number of 1740 cmxe2x88x921, which is ascribed to the stretching-vibration of a carbonyl group contained in the non-carbonate type ester linkage present in the polycarbonate resin, and A2 represents the absorption intensity at the wave number of 1780 cmxe2x88x921, which is ascribed to the stretching vibration of a carbonyl group of the carbonate type ester linkage present in the polycarbonate resin, each absorption intensity being measured by infrared spectroscopy. The non-carbonate type ester linkage described above is formed by heat decomposition of the polycarbonate resin. The particle size of the particulate heat deterioration product is the maximum diameter of the particulate heat deterioration product. The above-mentioned particulate heat deterioration product is considered to be formed by the decomposition of the polycarbonate resin which is caused by heat during the production of the polycarbonate resin. When the polycarbonate resin contains the particulate heat deterioration product in an amount larger than the above-mentioned range, the mechanical strength of a substrate for an optical information medium, which is produced by molding the polycarbonate resin, is likely to be lowered. However, with respect to the particulate heat deterioration products having a heat deterioration degree A1/A2 of less than 0.2, the influence of such products on the mechanical strength of the substrate is considered to be small. In the present invention, since there are some variations and errors in the position of the stretching vibration frequency obtained by infrared spectroscopy, the absorption intensity value at the top of the peak appearing within the wave number range of 1740xc2x115 cmxe2x88x921 (which is ascribed to the stretching vibration of a carbonyl group contained in the non-carbonate type ester linkage) is used as the absorption intensity A1, and the absorption intensity value at the top of the peak appearing within the wave number range of 1780xc2x115 cmxe2x88x921 (which is ascribed to the stretching vibration of a carbonyl group of the carbonate type ester linkage) is used as the absorption intensity A2.
In the present invention, it is preferred that the polycarbonate resin has terminal hydroxyl groups in an amount of from 5 to 50 mole %, based on the total molar amount of the terminal groups of the polycarbonate resin. The amount of the terminal hydroxyl groups is more preferably from 10 to 40 mole %, most preferably from 15 to 30 mole %, based on the total molar amount of the terminal groups of the polycarbonate resin. When the amount of the terminal hydroxyl groups is less than the above-mentioned range, it becomes difficult to obtain a substrate having a uniform quality, and when the amount of the terminal hydroxyl groups is more than the above-mentioned range, the stability of the polycarbonate resin during a high temperature molding thereof tends to be lowered. In general, the amount of the terminal hydroxyl groups can be determined by a method where the amount of the terminal hydroxyl groups is measured by NMR or a method where the amount of the terminal hydroxyl groups is calculated from the molar amount of the terminal hydroxyl groups and the total molar amount of the terminal groups, which are measured by titanium method, UV method, or IR method. However, in the present invention, the amount of the terminal hydroxyl groups is determined by the following method. A polycarbonate resin is dissolved in methylene chloride acidified with acetic acid and, then, titanium tetrachloride is added thereto to thereby form a red complex. The absorbance of the red complex is measured at 546 nm to determine the molar amount of the terminal hydroxyl groups (this method is the so-called titanium method). The total amount of the terminal groups is determined from the average number molecular weight of the polycarbonate resin which is obtained by GPC.
Hereinbelow, the method for producing the polycarbonate resin of the present invention is explained.
As mentioned above, the polycarbonate resin of the present invention is produced by subjecting an aromatic dihydroxy compound and a carbonic diester to a transesterification reaction.
In the present invention, the term xe2x80x9caromatic dihydroxy compoundxe2x80x9d means a compound represented by the formula:
HOxe2x80x94Arxe2x80x94OH
wherein Ar is as defined above. In the present invention, the aromatic dihydroxy compound may be a single type of aromatic dihydroxy compound or a combination of two or more types of aromatic dihydroxy compounds. It is preferred to use an aromatic dihydroxy compound in which the contents of a chlorine atom, an alkali metal and an alkaline earth metal are low. It is more preferred to use an aromatic dihydroxy compound substantially free of a chlorine atom, an alkali metal and an alkaline earth metal. Specifically, the content of a chlorine atom is preferably 0.5 ppm or less, and each of the contents of an alkali metal and an alkaline earth metal is preferably 0.1 ppm or less. The content of a chlorine atom can be measured by ion chromatography, and the contents of an alkali metal and an alkaline earth metal can be measured by atomic-absorption spectroscopy.
The carbonic diester used in the present invention is represented by the following formula: 
wherein each of Ar3 and Ar4 represents a monovalent C5-C200 aromatic group, and Ar3 and Ar4 may be the same or different.
In each of Ar3 and Ar4, which independently represents a monovalent carbocyclic or heterocyclic aromatic group, at least one hydrogen atom may be replaced by a substituent which does not adversely affect the transesterification reaction for producing the polycarbonate resin, such as a halogen atom, an alkyl group having from 1 to 10 carbon atoms, an alkoxy group having from 1 to 10 carbon atoms, a phenyl group, a phenoxy group, a vinyl group, a cyano group, an ester group, an amide group or a nitro group.
Representative examples of monovalent aromatic groups Ar3 and Ar4 include a phenyl group, a naphthyl group, a biphenyl group and a pyridyl group. These groups may or may not be substituted with the above-mentioned substitutent or substituents.
Preferred examples of monovalent aromatic groups as Ar3 and Ar4 include those which are respectively represented by the following formulae: 
Representative examples of carbonic diesters include di(unsubstituted or substituted)phenyl carbonate compounds represented by the following formula: 
wherein each of R9 and R10 independently represents a hydrogen atom, an alkyl group having from 1 to 10 carbon atoms, an alkoxy group having from 1 to 10 carbon atoms, a cycloalkyl group having from 5 to 10 ring-forming carbon atoms or a phenyl group; each of p and q independently represents an integer of from 1 to 5, with the proviso that when p is an integer of 2 or more, the R9""s are the same or different, and when q is an integer of from 2 or more, the R10""s are the same or different.
Of these diphenyl carbonate compounds, preferred are those having a symmetrical configuration, for example, di(unsubstituted)phenyl carbonate and di(lower alkyl-substituted)phenyl carbonates, e.g., ditolyl carbonate and di-t-butylphenyl carbonate. Particularly preferred is diphenyl carbonate which has the simplest structure.
These carbonic diesters may be used individually or in combination. It is preferred that these carbonic diesters have a low content of a chlorine atom, an alkali metal or an alkaline earth metal. It is most preferred that these carbonic diesters are substantially free of a chlorine atom, an alkali metal and an alkaline earth metal. Specifically, the content of a chlorine atom is preferably 0.5 ppm or less, and each of the contents of an alkali metal and an alkaline earth metal is preferably 0.1 ppm or less. The content of a chlorine atom can be measured by ion chromatography, and the contents of an alkali metal and an alkaline earth metal can be measured by atomic-absorption spectroscopy.
In the production of the polycarbonate resin of the present invention, the ratio of the aromatic dihydroxy compound to the carbonic diester (i.e., a charging ratio) varies depending on the types of the aromatic dihydroxy compound and carbonic diester employed, the polymerization temperature and other polymerization conditions, and the desired molecular weight of a polycarbonate resin to be obtained and the desired proportions of the terminal groups in the polycarbonate. The carbonic diester is generally used in an amount of from 0.9 to 2.5 moles, preferably from 0.95 to 2.0 moles, more preferably from 0.98 to 1.5 moles, per mole of the aromatic dihydroxy compound.
In the present invention, an aromatic monohydroxy compound or an aliphatic alcohol may be used for changing the terminal groups, or adjusting the molecular weight of the polycarbonate resin.
In the present invention, the production of a polycarbonate resin is conducted by a t transesterification process which is a process wherein a condensation polymerization of the aromatic dihydroxy compound and the carbonic diester is performed by transesterification in the molten state or solid state while heating in the presence or absence of a catalyst under reduced pressure, under an inert gas flow or under both reduced pressure and an inert gas flow. The mode of the transesterification process, the polymerization equipment and the like are not specifically limited. For example, when a molten-state transesterification is employed, examples of reactors employable for performing the transesterification reaction include an agitation type reactor vessel, a wiped film type reactor, a centrifugal wiped film evaporation type reactor, a surface renewal type twin-screw kneading reactor, a twin-screw horizontal agitation type reactor, a wall-wetting fall reactor, a free-fall polymerizer having a perforated plate, and a wire-wetting fall polymerizer having a perforated plate and at least one wire provided in association with the perforated plate. The transesterification reaction can be easily performed using these various types of reactors individually or in combination. Further, for example, the transesterification reaction can also be performed by a method in which a molten-state transesterification is first conducted to obtain a prepolymer, and the obtained prepolymer is then subjected to a solid-state polymerization under reduced pressure, under an inert gas flow or under both reduced pressure and an inert gas flow, using a solid-state polymerizer.
With respect to materials for constructing these polymerizers used in the present invention, there is no particular limitation. However, stainless steel, nickel or glass is generally used as a material for at least inner wall portions of polymerizers.
As an illustrative example of the method for producing the polycarbonate resin of the present invention containing specific amounts of heterounits (A) and (B), there can be mentioned a first method where heterounits (A) and (B) are introduced into the polycarbonate main chains using a specific aromatic dihydroxy compound or an aromatic poly(tri or more)hydroxy compound, which, when subjected to a transesterification reaction with a carbonic diester, form heterounits represented by formulae (2) and (3). Alternatively, the polycarbonate resin of the present invention can be produced without using the above-mentioned specific compounds, specifically, by a second method where recurring units (1) of the polycarbonate main chains are converted into the heterounits during the polymerization process by choosing appropriate polymerization conditions, such as polymerization temperature, type of catalyst, and residence time. The above-mentioned two methods can be used in combination. In the present invention, it is preferred to use the above-mentioned second method, since this method enables a easy production of a polycarbonate resin which not only does substantially not suffer occurrence of optical defects even under conditions wherein both temperature and humidity are high, but also exhibits an excellent balance between the mechanical properties and the low temperature moldability.
It is generally known in the art that, during the production of a polycarbonate resin by transesterification reaction, an aromatic dihydroxy compound undergoes a reaction similar to the Kolbe-Schmitt reaction under the influence of an alkali to thereby form a heterounit represented by the following formula (a): 
However, by regulating the reaction conditions, desired amounts of heterounit-(A) represented by the formula (2) and heterounit (B) represented by the formula (3) can be introduced into the polycarbonate main chains, while almost completely suppressing the formation of heterounit (a).
With respect to the above-mentioned second method for producing the polycarbonate resin of the present invention in which the heterounits are introduced by controlling the reaction conditions, it is preferred to use the method described in International Application Publication No. WO97/32916. As described in this publication, heterounits (A) and (B) are introduced into the polycarbonate main chains by controlling the relationship between the temperature and the residence time used for producing the polycarbonate resin. The longer the residence time of the polymerizable materials (i.e., a molten monomer mixture of an aromatic dihydroxy compound and a carbonic diester, and/or a molten prepolymer obtained by a process comprising reacting an aromatic dihydroxy compound with a carbonic diester) in a reaction zone having a high temperature, the larger the amounts of heterounits (A) and (B) introduced into the polycarbonate main chains. Especially, the amount of heterounit (B) introduced into the polycarbonate main chains tends to increase in accordance with the elevation of the reaction temperature. Therefore, desired amounts of heterounits can be introduced into the polycarbonate main chains by controlling the relationship between the temperature and the residence time of the polymerizable materials. In the present invention, in accordance with the technology described in the above-mentioned International Application Publication, a polycarbonate resin containing heterounits (A) and (B) in the above-mentioned specific amounts can be produced by strictly controlling the relationship between the temperature and the residence time used for producing the polycarbonate resin.
Specifically, it is preferred to employ a method in which at least one polymerizable material selected from the group consisting of a molten monomer mixture of an aromatic dihydroxy compound and a carbonic diester, and a molten prepolymer obtained by a process comprising reacting an aromatic dihydroxy compound with a carbonic diester, is subjected to a stepwise transesterification reaction, wherein the reaction conditions are controlled so as to satisfy the following formula (4):                     0.2        ≦                              ∑                          i              =              1                        n                    ⁢                      xe2x80x83                    ⁢                      (                          ki              xc3x97              Ti              xc3x97              Hi                        )                          ≧        1.2                            (        4        )            
wherein:
i represents the zone number assigned in an arbitrary order among n reaction zones of the reaction system,
Ti represents the average temperature (xc2x0 C.) of the polymerizable material in the i-th reaction zone,
Hi represents the average residence time (hr) of the polymerizable material in the i-th reaction zone,
ki represents a coefficient represented by the following formula (5):
ki=1/(axc3x97Tixe2x88x92b)xe2x80x83xe2x80x83(5)
wherein Ti is as defined above, and a and b depend on Ti, and wherein: when Ti satisfies the formula:
Ti less than 240xc2x0 C.,
a is 1.60046xc3x97105 and b is 0.472, when Ti satisfies the formula:
240xc2x0 C.xe2x89xa6Ti less than 260xc2x0 C.,
a is 4xc3x971049 and b is 19.107, and when Ti satisfies the formula:
260xc2x0 C.xe2x89xa6Ti,
a is 1xc3x9710122 and b is 49.082.
The value of       ∑          i      =      1        n    ⁢      xe2x80x83    ⁢      (          ki      xc3x97      Ti      xc3x97      Hi        )  
is preferably in the range of from 0.3 to 1.1, more preferably from 0.4 to 1.0.
Generally, in a process for continuously producing a polycarbonate resin by transesterification reaction, the transesterification reaction of the polymerizable material is stepwise conducted in a plurality of reaction zones, wherein the reaction temperature, residence time and reaction pressure are stepwise changed over the plurality of reaction zones involved in the process. The value of       ∑          i      =      1        n    ⁢      xe2x80x83    ⁢      (          ki      xc3x97      Ti      xc3x97      Hi        )  
in formula (4) represents the sum of the values of (kxc3x97Txc3x97H) for all of the reaction zones. For example, when a continuous polymerization is performed using a system in which a vessel for melting and mixing an aromatic dihydroxy compound and a carbonic diester, an agitation type reactor vessel, a centrifugal wiped film evaporation type reactor and a surface renewal type twin screw kneading reactor are serially connected through conduits,       ∑          i      =      1        n    ⁢      xe2x80x83    ⁢      (          ki      xc3x97      Ti      xc3x97      Hi        )  
is the sum of (kxc3x97Txc3x97H in the melting and mixing vessel), (kxc3x97Txc3x97H in the conduit connecting the melting and mixing vessel to the agitation type reactor vessel), (kxc3x97Txc3x97H in the agitation type reactor vessel), (kxc3x97Txc3x97H in the conduit connecting the agitation type reactor vessel to the centrifugal wiped film evaporation type reactor), (kxc3x97Txc3x97H in the centrifugal wiped film evaporation type reactor), (k xc3x97Txc3x97H in the conduit connecting the centrifugal wiped film evaporation type reactor to the surface renewal type twin screw kneading reactor), (kxc3x97Txc3x97H in the surface renewal type twin screw kneading reactor) and (kxc3x97Txc3x97H in the conduit connecting the surface renewal type twin screw kneading reactor to a nozzle for withdrawal of the produced polymer), that is, the sum of values of (kxc3x97Txc3x97H) for all of the reaction zones including the conduits. The term xe2x80x9ci-th reaction zonexe2x80x9d means a reaction zone falling on the number i which is determined by the numbering system in which all reaction zones including conduits, such as a mixing vessel, a reactor or a conduit which connect these apparatuses, are assigned their respective numbers in the arbitrary order. When a heater is disposed on a conduit connecting two reactors to each other, the conduit segment between one of the reactors to the heater, the heater, and the conduit segment between the heater and the other reactor are each regarded as a reaction zone. The average temperature of the polymerizable material means the average temperature of the polymerizable material in the i-th reaction zone. When the polymerizable material in the i-th reaction zone has a temperature distribution wherein different portions have distinctly different temperatures, each of such different portions may be separately regarded as an i-th reaction zone. With respect to the measurement of the average temperature, various methods may be employed. For example, the average temperature can be obtained by averaging one or more temperatures measured by one or more thermometers disposed at a reactor or a conduit. When no thermometers are disposed at a reactor or a conduit, the temperature of a heating medium in a jacket may be used as the average temperature. Alternatively, the average temperature of the inlet and outlet of a jacket for circulating a heating medium, or the temperature which has been set for a heater or a heating medium may be employed as the average temperature of the polymerizable material in the i-th reaction zone. The average residence time is calculated by dividing the volume of the polymerizable material held in the i-th reaction zone by the volume of the polymerizable material passing through or withdrawn from the i-th reaction zone per unit time.
The reaction pressure used for producing the polycarbonate resin of the present invention is preferably selected within the range of from atmospheric pressure to 1 mmHg, and the reaction pressure may vary depending on the reaction zone. In the system employed for the polymerization reaction, the reaction pressure of the polymerizer used for conducting the reaction in the final stage is preferably 5 mmHg or less, more preferably 3 mmHg or less.
A transesterification reaction can be carried out in the absence of a catalyst. However, if it is desired to accelerate the polymerization, the polymerization can be effected in the presence of a catalyst. The polymerization catalysts which are customarily used in the art can be used without particular limitation. Examples of such catalysts include hydroxides of an alkali metal and of an alkaline earth metal, such as lithium hydroxide, sodium hydroxide, potassium hydroxide and calcium hydroxide; alkali metal salts of, alkaline earth metal salts of and quaternary ammonium salts of boron hydride and aluminum hydride, such as lithium aluminum hydride, sodium boron hydride and tetramethyl ammonium boron hydride; hydrides of an alkali metal and of an alkaline earth metal, such as lithium hydride, sodium hydride and calcium hydride; alkoxides of an alkali metal and of an alkaline earth metal, such as lithium methoxide, sodium ethoxide and calcium methoxide; aryloxides of an alkali metal and of an alkaline earth metal, such as lithium phenoxide, sodium phenoxide, magnesium phenoxide, LiO-Ar-OLi wherein Ar represents an arylene group, and NaO-Ar-ONa wherein Ar is as defined above; organic acid salts of an alkali metal and of an alkaline earth metal, such as lithium acetate, calcium acetate and sodium benzoate; zinc compounds, such as zinc oxide, zinc acetate and zinc phenoxide; boron compounds, such as boron oxide, boric acid, sodium borate, trimethyl borate, tributyl borate, triphenyl borate, ammonium borates represented by the formula: (R1 R2 R3 R4)NB(R1 R2 R3 R4), and phosphonium borates represented by the formula: (R1 R2 R3 R4)PB(R1 R2 R3 R4), wherein R1, R2, R3 and R4 are as defined above; silicon compounds, such as silicon oxide, sodium silicate, tetraalkylsilicon, tetraarylsilicon and diphenyl-ethyl-ethoxysilicon; germanium compounds, such as germanium oxide, germanium tetrachloride, germanium ethoxide and germanium phenoxide; tin compounds, such as tin oxide, dialkyltin oxide, dialkyltin carboxylate, tin acetate, tin compounds having an alkoxy group or aryloxy group bonded to tin, such as ethyltin tributoxide, and organotin compounds; lead compounds, such as lead oxide, lead acetate, lead carbonate, basic lead carbonate, and alkoxides and aryloxides of lead or organolead; onium compounds, such as a quaternary ammonium salt, a quaternary phosphonium salt and a quaternary arsonium salt; antimony compounds, such as antimony oxide and antimony acetate; manganese compounds, such as manganese acetate, manganese carbonate and manganese borate; titanium compounds, such as titanium oxide and titanium alkoxides and titanium aryloxide; and zirconium compounds, such as zirconium acetate, zirconium oxide, zirconium alkoxide, zirconium aryloxide and zirconium acetylacetone.
The catalysts can be used individually or in combination. The amount of the catalyst used is generally in the range of from 10xe2x88x928 to 1% by weight, preferably from 10xe2x88x927 to 10xe2x88x921% by weight, more preferably from 10xe2x88x926 to 10xe2x88x922% by weight, based on the weight of the aromatic dihydroxy compound.
As mentioned above, the polycarbonate resin of the present invention is substantially free of a chlorine atom. Specifically, the content of a chlorine atom is {circle around (1)} 0.5 ppm or less as measured by potential difference titration or ion chromatography using a silver nitrate solution and, at the same time, {circle around (2)} 10 ppm or less as measured by combustion method for determining a chlorine atom, wherein 10 ppm is the detection limit of the combustion method for determining the chlorine atom. It is preferred that the content of a chlorine atom is 0.1 ppm or less as measured by method {circle around (1)} above, wherein 0.1 ppm is the detection limit of method {circle around (1)}, and, at the same time, 10 ppm or less as measured by method {circle around (2)} above. In the transesterification method, when a polycarbonate resin is produced from an aromatic dihydoxy compound and a carbonic diester which are both substantially free of a chlorine atom, the produced polycarbonate resin is also substantially free of a chlorine atom unless a chlorine-containing compound is added thereto.
In another aspect of the present invention, there is provided a polycarbonate resin composition for use in the production of a substrate for an optical information medium, comprising:
(I) 100 parts by weight of the polycarbonate resin; and
(II) 0.1xc3x9710xe2x88x924 to 100xc3x9710xe2x88x924 parts by weight of an acidic compound.
In general, it is known that the addition of an acidic compound to a polycarbonate resin tends to accelerate hydrolysis of the polycarbonate resin to thereby increase the opaqueness of the polycarbonate resin. The opaqueness of a polycarbonate resin is a phenomenon wherein the scattering of the light within the polycarbonate resin causes the polycarbonate to assume a white color, and is one of the optical defects The present inventors have studied this phenomenon by conducting the following experiment. A polycarbonate resin composition is produced by adding an acidic compound to a polycarbonate resin having an average molecular weight of approximately 2,300 to 2,600 (such a polycarbonate resin is generally used for injection molding). The produced polycarbonate resin composition is molded into a molded article having a thickness of approximately 3 mm. When the molded article is subjected to a steam test at 120xc2x0 C., it is observed that the degree of opaqueness of the molded article is very high as compared to that of a molded article produced from a polycarbonate resin alone without addition of an acidic compound. Therefore, it is very surprising that the addition of an acidic compound to the polycarbonate resin of the present invention, which contains specific amounts of heterounits (A) and (B) and which has a weight average molecular weight of 1,300 to 18,00, greatly suppresses the occurrence of optical defects under conditions wherein both temperature and humidity are high.
There is no particular limitation with resect to the acidic compound contained in the polycarbonate resin composition of the present invention, except that phenolic compounds which is formed during the production of the polycarbonate resin are excluded from the acidic compound used in the present invention. As the acidic compound, compounds having pKa of 5 or less (solvent: water, or a mixed solvent of water and methanol) are preferably used. Examples of acidic compounds include inorganic acids, such as phosphoric acid, phosphorous acid, hypophosphorous acid, pyrophosphoric acid and boric acid; organic acids, such as adipic acid, citric acid and acetic acid; sulfonic acids, such as benzenesulfonic acid and p-toluenesulfonic acid; and sulfonic esters, such as ethyl benzenesulfonate and butyl p-toluenesulfonate. Of these, especially, phosphoric acid, citric acid and sulfonic esters are preferably used. The resin composition of the present invention contains an acidic compound in an amount of from 0.1xc3x9710xe2x88x924 to 100xc3x9710xe2x88x924 parts by weight, preferably from 0.4xc3x9710xe2x88x924 to 50xc3x9710xe2x88x924 parts by weight, more preferably 0.8xc3x9710xe2x88x924 to 20xc3x9710xe2x88x924 parts by weight, relative to 100 parts by weight of the polycarbonate resin. When the polycarbonate resin composition contains an acidic compound in an amount outside the above-mentioned range, a substrate for an optical information medium produced from such a polycarbonate resin composition suffers the optical microdefects under conditions wherein both temperature and humidity are high. With respect to the method for producing the polycarbonate resin composition of the present invention, the polycarbonate resin composition can be produced by a method in which an acidic compound is mixed with the polycarbonate resin immediately after the production thereof by the melt polymerization, while the polycarbonate resin is still in the molten state. Alternatively, the polycarbonate resin composition can be produced by a method in which an acidic compound is mixed with the pelletized polycarbonate resin and the resultant mixture is subjected to a melt-kneading. The amount of an acidic compound contained in the polycarbonate resin composition can be determined using conventional methods, such as NMR, atomic absorption, and liquid chromatography.
The resin composition of the present invention may optionally contain conventional additives other than the above-mentioned acidic compound in amounts as generally employed in the art. Examples of such conventional additives include a thermal stabilizer, an antioxidant, a weathering stabilizer, a UV light absorber, a mold release agent, a lubricant, an antistatic agent and a plasticizer. With respect to a method for introducing the conventional additive into the polycarbonate resin composition, the conventional additive may be mixed with the polycarbonate resin immediately after the production thereof, while the polycarbonate resin is still in the molten state. Alternatively, the conventional additive may be mixed with the pelletized polycarbonate resin or the pelletized polycarbonate resin composition, followed by subjecting the resultant mixture to a melt-kneading.
In still another aspect of the present invention, there is provided a substrate for an optical information medium, which is produced by molding the polycarbonate resin or polycarbonate resin composition of the present invention.
In the present invention, a xe2x80x9csubstrate for an optical information mediumxe2x80x9d means a substrate used for producing an optical information medium which is capable of optically recording and providing digital information using microgrooves and micropits formed on the substrate thereof. Specific examples of optical information media include, a CD, a CD-ROM, a CD-R, a CD-RW, a DVD-ROM, a DVD-R, a DVD-RW, an MD and an MO. In the present invention, there is no particular limitation with respect to a method for producing the substrate for an optical information medium. For example, the substrate may be produced by a method comprising the following steps (1) to (3):
(1) producing the above-mentioned polycarbonate resin or polycarbonate resin composition of the present invention;
(2) introducing the produced polycarbonate resin or polycarbonate resin composition into an injection molding machine designed for producing optical disks; and
(3) subjecting the polycarbonate resin or polycarbonate resin composition to an injection molding under conditions wherein the molding temperature is 300 to 370xc2x0 C., the mold temperature is 50 to 130xc2x0 C., and the molding cycle is 3 to 10 seconds.
As the injection molding machine used in this method, a conventional injection molding machine designed for producing optical disks can be used. The substrate of the present invention exhibits not only a high precision transferability (i.e., information of a stamper (a mold for forming microgrooves and micropits on a substrate) can be transferred to the substrate with high precision), but also a low birefringence. In addition, the substrate of the present invention not only does substantially not suffer occurrence of optical defects which cause an error in reading the digital information from the optical disk or an error in writing the digital information onto the disk, but also exhibits excellent mechanical strength and resistance to moist heat. By the virtue of these properties, the substrate of the present invention is suitable for producing an optical information medium capable of retaining the stored information for a prolonged period of time.
The present invention will be further described in more detail with reference to the following Examples and Comparative Examples, which should not be construed as limiting the scope of the present invention.